Multi-carrier operation improves achievable throughput and coverage of wireless access systems. In multi-carrier operation, a wireless transmit/receive unit (WTRU) may be configured and activate more than one frequency carrier in the uplink (UL) and/or in the downlink (DL). The multi-carrier operations would allow UL and DL transmission bandwidths to exceed a single carrier frequency and allow for more flexible and more efficient usage of the available spectrums.
For flexible and efficient usage of the available spectrums and for effective support for the asymmetric traffic loads in the DL, the multi-carrier configuration with an unpaired DL carrier(s) has been proposed. An unpaired DL carrier is a DL carrier that does not have a corresponding UL carrier. For example, in frequency division duplex (FDD) systems, the DL may contain a first 20 MHz carrier and a second 10 MHz carrier and the UL may have a 20 MHz carrier. In this example, the second DL 10 MHz carrier, which does not have a paired UL carrier, is an unpaired DL carrier. The unpaired DL carrier may occur in time division duplex (TDD) systems as well. For example, a subscriber may have a first carrier activated on both the DL and the UL and a second carrier activated only on the DL, where the second carrier with only DL activation is an unpaired DL carrier. Another example of unpaired DL carrier is a partially configured carrier, which is defined as DL-only transmission carrier in TDD or a DL carrier without paired UL carrier in FDD.